In French Pat. No. 1,582,543 in the name of the applicants, there is a description of catalysts for the polymerization of olefins comprising a catalytic solid obtained by reacting a halogenating agent and a derivative of a transition metal with a solid support consisting of an oxygenated compound of a divalent metal in the anhydrous state and low in hydroxyl groups. The oxygenous compounds preferably used are oxides. The reaction conditions of the solid support with the halogenating agent are selected in such a way as to fix such a quantity of halogen that the atomic ratio of halogen to divalent metal is less than 1. While satisfactory, such catalysts are not as active as desired.